The present invention relates to semiconductor devices and apparatus for their manufacture. More particularly, the present invention relates to improved apparatus and methods for clamping a semiconductor substrate on a bipolar electrostatic chuck in a plasma processing chamber.
The use of bipolar electrostatic chucks in plasma processing systems is well known. To facilitate discussion of the foregoing, FIG. 1 illustrates a simplified schematic of a substrate processing chamber which represents a chamber suitable for use with a bipolar electrostatic chuck. Referring to FIG. 1, a substrate plasma processing system 100 includes a plasma processing chamber 110. Within chamber 110, there is disposed an electrode 104 which may represent a showerhead-type or TCP coil is energized by a RF generator 108.
Within the top portion of chamber 110 and extending into the top portion of electrode 104, there is provided a gas orifice 102 into which gaseous source materials (e.g., the etchant source gases) are directed. In this embodiment, showerhead-type electrode 104 preferably includes a plurality of baffles 106 for diffusing gaseous source materials into the RF-induced plasma region 134 above a substrate 112, representing for example a semiconductor substrate or a flat panel display. The gaseous source materials may also be released from ports built into the walls of the chamber itself or from a gas ring apparatus disposed below the substrate.
Substrate 112 is introduced into chamber 110 and disposed on a bipolar electrostatic chuck 114. Bipolar electrostatic chuck 114 is integrally attached and electrically connected to a lower electrode 116. Bipolar electrostatic chuck 114 and lower electrode 116 are therefore typically at the same RF potential. Lower electrode 116 typically receives RF power from RF generator 108. A power supply 170 provides a negative bias voltage V.sub.N by way of a lead 123 to a first buried plate 152 as well as a positive bias V.sub.p by way of a lead 124 to a second buried plate 150.
To protect power supply 170 from the RF energy supplied by RF generator 108, RF filters (conventional and omitted from FIG. 1 to simplify the illustration) may be disposed between power supply 170 and RF generator 108. Analogously, dc blocking capacitors (conventional and omitted from FIG. 1 to simplify the illustration) may also be disposed between power supply 170 and RF generator 108 to prevent RF generator 108 from being affected by the dc potential levels supplied by power supply 170.
A coolant port 122 extends through both lower electrode 116 and electrostatic chuck 114. Helium cooling gas is introduced under pressure (e.g., about 5-10 Torr in one embodiment) via coolant orifice 122. The helium cooling gas impinges a lower surface of substrate 112 to act as a heat transfer medium for accurately controlling the substrate's temperature during processing to ensure uniform and repeatable process results. During plasma processing, the pressure within chamber 110 is preferably kept low by withdrawing gas through a port 160, e.g., between about 5 to 25 mTorr in one embodiment. A plurality of heaters (omitted from FIG. 1 to simplify the illustration) may be provided to maintain a suitable chamber temperature for etching (e.g., about 70.degree. C. in one embodiment). To provide an electrical path to ground, the wall 111 of chamber 110 may be typically grounded.
FIG. 2A illustrates in greater detail a cross-section of bipolar electrostatic chuck 114 of FIG. 1. As the name implies, bipolar electrostatic chuck 114 has two buried plates: a negatively charged buried plate 152 and a positively charged buried plate 150. The buried plates of bipolar electrostatic chuck 114 are coupled to power supply 170 of FIG. 1. When power supply 170 is turned on, buried plate 150 is positively biased by power supply 170 relative to the common reference potential level. Power supply 170 also biases buried plate 152 negatively relative to the common reference potential level. Since the buried plates 150 and 152 are in fixed locations, the electrostatic fields created by the charged buried plates are well defined in the region surrounding each buried plate.
For an p-type semiconductor wafer, holes in substrate 112 migrate toward a region of substrate immediately overlying negatively biased buried plate 152. A region of net positive charge is thereby formed equal in field strength but opposite in polarity to the field created in electrostatic chuck 114 by buried plate 152. A resulting region of net negative charge is formed in substrate 112 in the area immediately overlying the positively biased buried plate 150. In the same manner, for an n-type semiconductor wafer, electrons in substrate 112 migrate toward the region of substrate 112 immediately overlying positive buried plate 150. A region of net negative charge is thereby formed equal in field strength but opposite in polarity to the field created in electrostatic chuck 114 by buried plate 150. A resulting region of net positive charge is formed in substrate 112 in the region immediately overlying the negatively biased buried plate 152.
As is known to those skilled in the art, the presence of the oppositely charged regions in substrate 112 and the biased plates 150 and 152 in electrostatic chuck 114 results in induced electrostatic forces coupling substrate 112 and electrostatic chuck 114. By way of example, FIG. 2A shows an electrostatic force F.sub.1 between positively charged plate 150 and the negatively charged region 122 of substrate 112. FIG. 2A also shows an electrostatic force F.sub.2 between negatively charged plate 152 and positively charged region 124 of substrate 112. As is known in the art, the strength of force F.sub.1 is directly proportional to the potential difference between biased plate 150 and charged region 122. In a similar manner, the strength of force F.sub.2 is directly proportional to the potential difference between biased plate 152 and charged region 124. In this example, electrostatic forces F.sub.1 and F.sub.2 act to clamp substrate 112 to electrostatic chuck 114.
Typically, it is desirable to have balanced clamping forces (i.e., electrostatic force F.sub.1 and electrostatic force F.sub.2 are substantially equal in strength) applied to a workpiece, or substrate, during the plasma operation.
However, subsequent to the introduction of a negatively charged plasma into chamber 110, substrate 112 becomes negatively biased with reference to the electrostatic chuck by plasma induced bias -V.sub.B. By way of example, FIG. 2B is an illustration of the effect of a negatively charged plasma 180 on the electrostatic clamping forces F1 and F2. The plasma induced bias -V.sub.B in substrate 112 offsets the relative voltage potential drops between the induced charge regions 122 and 124 in substrate 112 and biased buried plates 150 and 152 (which are held at a fixed potential relative to a common ground by power supply 170). Since electrostatic forces F.sub.1 and F.sub.2 are directly proportional to the relative potential differences between regions 122 and 124 and plates 150 and 152, the plasma induced bias -V.sub.B will unbalance clamping forces F.sub.1 and F.sub.2 by increasing force F.sub.1 and decreasing force F.sub.2.
To illustrate this condition, consider the case where power supply 170 biases positive pole 204 at +350 V and negative pole 206 at -350 V relative to the common reference voltage level. With the plasma off, the substrate potential is at 0 V relative to the common reference voltage level, and the potential differences between the poles of bipolar chuck 114 and their overlying substrate regions are +350 V and -350 V respectively.
When substrate 112 is negatively charged due to the presence of plasma, however, the potential differences between the substrate and the two poles of the bipolar electrostatic chuck may become asymmetric. For example, the substrate bias voltage may be -100 V when the plasma is turned on. In this case, the potential difference between the positive pole and the negatively biased substrate is increased to +450 V, i.e., (+350 V- (-100 V)). However, the potential difference between the negative pole and the negatively biased substrate is decreased to only -250 V, i.e., (-350 V- (-100 V)). The reduction in the potential difference reduces the electrostatic holding forces between the negative pole and the wafer. Consequently, some heat-exchange gas may escape, resulting in inadequate temperature control and/or process variations. In some cases, the electrostatic force holding the substrate to the bipolar chuck may become so weak that it is insufficient to resist the force exerted on the substrate by the pressure differential between the helium cooling pressure and the low pressure within the chamber, resulting in the substrate "popping off" the chuck's surface.
Further, the plasma-induced negative substrate bias may unduly increase the potential difference between the negatively-biased substrate and the positive pole of the bipolar chuck. An excessively high potential difference may cause arcing ( i.e., sparking) between the lower surface of the substrate and the upper surface of the chuck or excessive current into or out of the plasma, resulting in pit mark damage. Over time, the surface of the chuck may be damaged to the point where it becomes impossible to keep the heat-exchange gas properly sealed.
A prior art attempt to compensate the plasma induced bias is illustrated in FIG. 3. The electrostatic chuck 414 includes a negatively biased buried plate 420 and a positively biased plate 418. A DC power supply 406 and a DC power supply 408 supply the necessary voltage potential with respect to a variable ground reference node 407 to plates 418 and 420, respectively, to create regions of opposite and equal charges in substrate 416. These regions of opposite and approximately equal charges create the required clamping forces. A pickup pin 402 is disposed inside the chamber to sense the plasma induced bias created in wafer 416 and, in this manner, provide the input necessary to vary reference node 407. Varying ground reference node 407 has the effect of compensating for the induced plasma bias by appropriately increasing and decreasing the applied potentials to plates 418 and 420.
However, the use of pick up pin 402 has many disadvantages. One such disadvantage is the fact that the pick up pin 402 is only approximating the plasma induce bias in substrate 416. Unfortunately, this approximation may be affected by many factors not under the control of the user. By way of example, pick up pin 402 will, overtime, undergo plasma induced damage known in the art as sputtering in which the energetic plasma ions cause particles of the pick up pin to dislodge and contaminate the plasma within the chamber. In addition to introducing unwanted contamination, this damage will alter the electrical characteristics of the pick up pin in a manner which requires routine calibration and adjustment (i.e., presence of resistor bridge 450 to compensate for variation in electrical characteristics of pick up pin 402) as well as eventual replacement resulting in significant downtime and lost production.
Pick up pin 402 may not be able to adequately compensate for the plasma induced bias in substrate 416 if the plasma in the region where it is located is significantly different than that impinging on the surface of substrate 416. This difference in plasma between that seen by pick up pin 402 and wafer 416 may be due to such factors as: spatial variations ( i.e., radial distribution of plasma density/temperature), non-uniformities in the plasma itself due to irregular geometries, for example. In practice, any one of these factors may render use of pick up pin 402 as a mechanism for compensating for the plasma induced bias less than satisfactory.
In view of the foregoing, there are desired improved techniques for improved clamping of the substrate to the bipolar electrostatic chuck, particularly in the presence of plasma. To lower cost, and/or reduce contamination, the improved techniques preferably do not require the use of a pick-up pin, and/or a second power supply or complicated control circuitries.